To determine whether extracellular PDl plays a role in thrombus formation, PDl expression, platelet accumulation, and fibrin generation were monitored in the blood vessels of mice following laser-induced arteriolar injury. A time-dependent increase in PDl antigen was observed in the thrombus following injury. Infusion of bacitracin or a blocking monoclonal antibody to PDl into the circulation completely inhibited platelet thrombus formation and fibrin generation. These results indicate that PDl is required in vivo for both fibrin generation and platelet thrombus formation. The objective in this project is to determine whether inhibitors of PDl represent a new class of antithrombotic agents. In Aim #1, the current project proposes to Identify the substrates of PDl that participate in the initiation of thrombus formation by the expression of mutant forms of thiol isomerases into experimental thrombi ex vivo and the trapping of disulfide-linked complexes of thiol isomerases and their substrates. In Aim #2, the xray crystal structure of human PDl and PDl in complex will be determined. These structures will include PDl and PDl bound to an anfi-PDI Fab, PDl complexed to quercetin 3-rutinoside and to quercetin, and PDl complexed to beta3 integrin. In Aim #3, antithrombotic properties of quercetin will be tested in mice. The pharmacology of quercetin as an antithrombotic agent will be initially studied by direct visualization of thrombus inhibifion using the laserinduced thrombus formation model in the cremaster muscle of the mouse using intravital microscopy. The inhibition of thrombosis-initiated fetal/neonatal demise by quercetin will be tested in two mouse models: perinatal thrombosis with homozygous Factor V Leiden and heterozygous TFPI deficiency, and consumptive coagulopathy with homozygous protein C deficiency. If quercetin is an active antithrombotic, we will determine whether quercetin, which inhibits both platelet aggregation and fibrin generation, is superior to standard antithrombotic agents. PDl inhibitors will be compared to conventional antithrombotic agents in mouse models of thrombosis. These will include death or paralysis induced by pulmonary embolism, and by intravital study of thrombus formation associated with acquired anti-phospholipid syndrome.